This invention relates to a 4-high rolling mill of the work roll shift type, and more particularly to a 4-high rolling mill which has an excellent ability of controlling a plate crown and a plate shape of a material to be rolled, and enables a schedule-free rolling.
In recent years, the functions required for a rolling mill (particularly, a hot strip mill) are a schedule-free rolling and a rolling for highly precisely controlling a plate crown and a plate shape of a material to be rolled. The term "schedule-free rolling" means the type of rolling in which any desired width of the material to be rolled can be freely selected, with no limitation imposed on the order of selection of widths of the material.
The type of mill capable of achieving this function is known as HCW mill as disclosed in Japanese Patent Examined Publication No. 51-7635. In this system, the plate crown and plate shape of the material to be rolled are controlled by the shift of intermediate rolls and work roll benders, and wear of the roll surface is dispersed by a cyclic shift of the work rolls, thereby achieving the schedule-free rolling. Therefore, it is inevitable for this mill to be of the 6-high type requiring the intermediate roll shift and the work roll shift. In a finish mill (hot strip mill) of the tandem type, a later-stage stand requires a small torque, and also the plate thickness is small, and therefore the size of the rolling mill is not unduly increased even if the 6-high rolling mill is used, because the diameter of the work rolls can be made small. However, a preceding-stage stand requires the work rolls of a large diameter, and therefore if the 6-high rolling mill is used, its size becomes enormous.
Therefore, this function must be achieved by a 4-high rolling mill. If the system disclosed in Japanese Patent Examined Publication No. 51-7635 is applied to a 4-high rolling mill, it is necessary to move an end of the effective length of the roll barrel to a position near an end of the width of the material to be rolled in order to decrease the plate crown, and in this condition the rolling is carried out. Therefore, when the material to be rolled is displaced from the center of the mill, there occurs a disadvantage that the material to be rolled is disengaged from the barrels of the work rolls. Further, in this system, when the work rolls are reciprocally moved (that is, shifted in a cyclic manner) in order to disperse wear of the work rolls, the end of the roll barrel may be spaced more than 200 mm from the lateral edge of the material to be rolled, because the amplitude of this cyclic shift is about .+-.100 mm. At this time, any extra force for amending the plate crown does not already remain in a roll bender at all. If a roll crown is formed on the peripheral surface of the roll barrel, this difficulty can be overcome; however, this roll crown is limited to a slightly convex shape in order to prevent the plate crown from having a concave shape when the width of the material is wide. Therefore, the plate crown can not be effectively controlled when the material width is small. When a decrease bender is used, it is possible to increase the roll crown. However, the decrease bender must be switched to a roll balance when the material is passed between the work rolls, and therefore there is encountered a disadvantage that the passage of the material through the work rolls is unstable. Further, in this HCW mill system, the load of contact of the end portion of the work roll barrel with the backup roll is high, and the lifetime of the backup roll is shortened particularly in a heavy-load rolling. Therefore, generally, the HCW mill is conventionally used to deal with the wear of the rolls.
Japanese Patent Unexamined Publication No. 57-91807 discloses a mill of the work roll shift type. An S-shaped concave-convex roll crowns are formed on peripheral surfaces of the barrel of work rolls, and the upper and the lower work roll are disposed in reverse relation to each other (In other words, one of the upper and the lower work roll is turned through 180.degree. relative to the other). In this mill, the work rolls are shifted so as to geometrically change the shape of a roll gap between the two work rolls in the axial direction of the work rolls. A feature of this mill is that a change of the plate crown relative to the shift amount is large. In this case, a backup roll barrel and the work roll barrel are in contact with each other generally over their entire lengths, and therefore a great effect of a work roll bender as achieved in the HCW mill cannot be expected.
In this type of mill, when the work rolls are cyclically shifted so as to disperse wear of the rolls, the plate crown is greatly varied. The amount of shift of the work roll in the above-mentioned mill is around .+-.100 mm, so that the plate crown is changed from the maximum to the minimum. On the other hand, when the work roll is cyclically shifted by an amount of .+-.100 mm in order to achieve the roll wear dispersion, the plate crown is cyclically varied. This plate crown variation cannot be amended by such a work roll bender having a small effect.
Namely, the mill of the above type cannot effect a schedule-free rolling, though it has a plate crown control ability.
Another 4-high rolling mill having a large plate crown control amount is one called "a pair cross mill" as disclosed in Japanese Patent Unexamined Publication no. 55-64908. In this type of mill, upper and lower work rolls, as well as backup rolls, are disposed horizontally, with their axes intersecting each other, so that the profile of the amount of a vertical roll gap between the work rolls can be changed so as to control the plate crown. In this type of mill, if only the work rolls are disposed in intersecting relation to each other, a slip occurs between the work roll and the backup roll, so that a roll wear and a large thrust are produced. To avoid this, it is necessary that the backup rolls for receiving the rolling load should also be disposed in intersecting relation to each other. As a result, the mill has a large and complicated construction. Further, a spindle for driving each work roll is angularly moved in accordance with a vertical position change of the work roll, and also is inclined in a horizontal direction for the intersection of the work rolls, so that the overall angle of each spindle is increased. Therefore, a universal joint suited for such a large angle change is needed. However, since the rotational speed is changed in accordance with the above horizontal inclination angle, a gear-type spindle suited for a small angle change must be used, and therefore the intersection angle is limited. Further, in order that this pair cross mill can achieve a schedule-free rolling, it is very important how wear of the work roll can be dealt with.
One means for dealing with this problem to add the function of shifting each work roll in its axial direction. With this arrangement, however, the axial shift mechanism is further added to the horizontally-intersecting work rolls, so that the construction becomes extremely complicated. Such a mill, used in a severe environment in which the load, the impact, the heat and water are applied, is not satisfactory in reliability and maintenance.
Another means for dealing with the above problem is to provide roll grinders in a rolling mill, as disclosed in Japanese Patent Unexamined Publication No. 54-145356. When a work roll is subjected to wear, the peripheral surface of the work roll barrel is ground by the roll grinder so that the roll crown can be always kept to the same shape before the wear. In this mill, many roll grinders for applying a large grinding load are required so as to sufficiently compensate for the wear of the work rolls. As a result, the size of the rolling mill is increased, and also the cost for the maintenance such as the exchange of many whetstones is increased.
Japanese Patent Unexamined Publication No. 57-181708 discloses a 4-high rolling mill provided with work rolls shiftable in the direction of the axis thereof. Each of the work rolls has a convex initial crown formed axially over less than a half of its length, and the two work rolls are so arranged that their convex initial crown portions are disposed oppositely relative to each other. Each of the backup rolls either has a convex initial crown which is formed over the entire length thereof and is symmetrical with respect to the center of its length, or has a convex initial crown which extends over less than a half of its length and is disposed oppositely relative to the initial crown of the work roll.
The two work rolls are shifted in opposite directions in accordance with the width of a material to be rolled, and each of the lateral edges of the material to be rolled is positioned between the initial crown portion of one of the work rolls and the cylindrical portion of the other work roll, and in this condition the rolling is carried out. During the rolling, the pressure applied to the lateral edge portions of the material to be rolled is reduced by the initial crowns of the work rolls, thus controlling the edge drop of the material to be rolled. Also, the contact pressure between the work rolls and the backup rolls are reduced by the initial crown of the backup rolls, thus controlling the plate crown of the material to be rolled. By combining these two effects, the plate crown and the plate shape are controlled.
As described above, in this rolling mill, when the rolling is carried out, each of the lateral edges of the material is positioned between the initial crown portion of one work roll and the cylindrical portion of the other work roll. In other words, the major portion of the material to be rolled is rolled between the straight portions of the work rolls. Therefore, it is difficult to control the plate crown at these portions. The contact pressure between the work rolls and the backup rolls is excessive, so that the wear of the straight portion becomes large. As a result, the plate crown and the plate shape are not controlled satisfactorily, and also the schedule-free rolling cannot be effected.
Generally, the backup rolls are not mounted on the rolling mill in such a manner that they are frequently exchanged. Therefore, the backup rolls having the initial crown must be used for a long period of time, and the initial crown of the backup rolls cannot be maintained. As a result, it is difficult to maintain a high-precision control of the plate crown. If the backup rolls are frequently exchanged, the time of stop of the rolling mill required for the exchange becomes long, and the production efficiency of the rolling mill is lowered. Further, it is necessary to provide such a construction as to facilitate the exchange of the backup rolls.